Automotive fuel injectors for delivering fuel into a combustion space of an internal combustion engine typically comprise a control piston which is operable to control the pressure of fuel contained within an injector control chamber. The control chamber is defined, in part, by a surface associated with an injector valve needle such that changes of fuel pressure within the control chamber cause the valve needle to move into and out of engagement with an associated seating surface. As a result, delivery of pressurised fuel into the combustion space of the engine is controlled.
It is known to provide a fuel injector with a piezoelectric actuator to control movement of the control piston. Piezoelectric actuators used for this purpose are generally in the form of a stack body having a multi-layered laminated stack of piezoceramic elements or layers, each of which is separated by an electrically conductive layer. The electrically conductive layers act as internal electrodes. The stack body is provided with positive and negative distribution electrodes arranged to establish electrical contact with the internal electrode layers. The distribution electrodes themselves are connectable to a power source, thus conveying a voltage to the internal electrode layers.
Application of a voltage across the internal electrode layers causes the stack to expand or contract in dependence on the polarity of the applied voltage. Accordingly, the magnitude of the expansion and contraction corresponds to the magnitude of the applied voltage. The change in length of the stack is used to control movement of the control piston, thus controlling opening and closing of the valve needle.
It is known that the application of a compressive force to a piezoelectric stack influences beneficially its performance and durability. In the case of piezoelectrically operable fuel injectors for use in compression-ignition internal combustion engines, a known approach is to locate the piezoelectric stack within a fuel-filled fluid volume or chamber. Typically, fuel within the fluid chamber is maintained at pressures of up to 2000 bar and so the piezoelectric stack experiences a high compressive loading due to hydrostatic forces. An actuator of this type is exemplified in EP0995901. Enveloping the stack within a polymer casing or sleeve guards against ingress of high pressure fuel into the stack structure.
Although locating the piezoelectric actuator in a high pressure fluid environment has advantages, it is necessary to ensure that the electrical connections to the stack are adequately and reliably sealed from high pressure fuel within the fluid chamber. Also, it is vital to ensure that the high hydrostatic forces and the aggressive effects of the fuel-laden environment do not adversely affect the provisions for insulating the stack.
It is an object of the present invention to provide an improved electrical connector arrangement for such a piezoelectric actuator which addresses these issues.